1. Field of the Invention
The present invention relates to a projection system and, more particularly, to an illuminating system and method for improving asymmetric projection in a projection system.
2. Description of Related Art
Referring to FIG. 1, a projection system 20 in a prior art is a single panel full color system having a light source 21 with a parabolic concave reflector 211 to emit a white light beam 23. After reflected by the parabolic concave reflector 211, the white light beam 23 passes through a converging lens 22 to converge the light beam 23 on a color wheel 24. When rotating around a shaft, the color wheel 24 has a sequence of red, green, and blue filters to intercept the light beam 23 in order, so the light beam 23 shifts into a color light beam 25 as passing through the color wheel 24. Then, the color light beam 25 transmits in order through an integration rod 26, a condenser lens 27, a stop 28, and a relay lens 29 to impinge in a prism 30, and a mirror 31 in the prism 30 reflects the color light beam 25 to a light valve 10, such as Digital Micro-mirror Device (hereinafter referred to as “DMD”). Therefore, the light valve 10 modulates the color light beam 25 to generate an image light and reflects the image light into a projection lens 32 for projecting an image onto a screen (not shown).
The DMD has an array of inclinable mirrors corresponding to an array of pixels constituting the image. When one of inclinable mirrors reflects a light beam to the screen, the inclinable mirrors is referred to as “on-state”; when reflecting a light beam off the screen, the pixel mirror is referred to as “off-state”; when the pixel mirror parallel the DMD board, that is between the on-state and the off-state, the pixel mirror is referred to as “flat-state”.
FIG. 2 shows a second projection system 40 in a prior art. The difference, between the first projection system 20 and the second projection system 40, is that the light source 41 of the second projection system 40 has an elliptic concave reflector 411. The elliptic concave reflector 411 reflects and converges a light beam, emitted from the light source 41, to a color wheel 44 without passing a converging lens. For the rest, the second projection system 40 is the same as the first projection system 20 and actually is one embodiment of the first projection system 20. To simplify the description, the first projection system 20 was only described as an example in the following specification, buts its technologies can be fully suitable for the second projection system 40.
In the first projection system 20, from the light source 21 to the light valve 10, the light beam passes through any optical element, such as the converging lens 22, the color wheel 24, integration rod 26, the condenser lens 27, the stop 28, and the relay lens 29, having the geometrically symmetric feature. Therefore, as shown in FIG. 3, before obliquely impinging onto the light valve 10, the light beam has a rectangular cross section to form a rectangular-sectional light beam 51 with a feature of an intensely uniform illumination. If any above-mentioned optical elements and their assembly are perfect, the length of a first diagonal L1 will be equal to that of a second diagonal L2 on the rectangular-sectional light beam 51, i.e. the rectangular-sectional light beam 51 has no distortion.
Referring to FIG. 4, when the rectangular-sectional light beam 51, passing through the prism 30, obliquely impinges onto the light valve 10, a light spot 52 is formed thereon. Consequently, the light spot 52 is distorted due to the inclined incidence, so that the length of the first diagonal L1 isn't equal to that of the second diagonal L2, i.e. L1>L2.
The above-mentioned prior art has at least two drawbacks as follows: one reduces the uniform brightness of the light spot resulted from the unequal extension of two diagonals; the other causes the brightness loss on circumference of the light spot from the extension of the first diagonal L1 out of the light valve and results in decreasing the illumination efficiency.